#include <cstdio>

using namespace std;
char v[10001];
int f[300];
int main()
{
    FILE *fin=fopen ("litere.in","r");
    FILE *fout=fopen ("litere.out","w");
    int n,i,j;
    long long sol;
    char c;
    fscanf (fin,"%d\n",&n);
    c=fgetc(fin);
    i=1;
    while (c!='\n'){
        v[i]=c;
        c=fgetc (fin);
        i++;
    }
    sol=0;
    for (i=1;i<=n;i++){
        for (j=v[i]+1;j<='z';j++)
            sol+=f[j];
        f[v[i]-'a'+97]++;
    }
    fprintf (fout,"%lld",sol);
    return 0;
}